Spectrometer with rotary scanning disc and method of spectroscopic analysis

ABSTRACT

A spectrometer and method of spectroscopic analysis for providing spectral data concerning a scene by collecting and collimating radiation from the scene to provide a radiation beam dispersing the radiation of the beam into a spectrum, forming a focused image of the spectrum, scanning the spectrum image in rapid succession by multiple narrow radiation receiving areas, and generating an electrical output representing the radiant flux incident on each area during its relative scanning movement along the image. The spectrometer may be equipped with means for mounting radiation filters over selected radiation receiving areas to provide information concerning selected characteristics, such as relative polarization, of the radiation from the spectrum image and for mounting narrow band-pass calibration filters over alternate radiation receiving area to permit calibration of the spectral data obtained from the scene.

United States Patent White [54] SPECTROMETER WITH ROTARY SCANNING DISCAND METHOD OF SPECTROSCOPIC ANALYSIS [72] Inventor: Peter G. White,Torrance, Calif.

[73] Assignee: TRW Inc., Redondo Beach, Calif.

[22] Filed: Dec. 29, 1969 [21] Appl. No.: 888,385

[52] U.S. Cl. ..356/83, 356/96, 356/99 [51] Int. Cl. ..G01j3/42, GOlj3/12 [58] Field of Search ..350/273, 275; 356/74, 101

[56] References Cited UNITED STATES PATENTS 2,010,307 8/1935 Leishman..350/273 2,750,836 6/1956 Fastie ..356/ 100 2,823,577 2/1958 Machler..356/86 2,946,879 7/1960 Powers ..356/99 2,995,973 8/1961 Barnes et a]...356/99 3,460,892 8/1969 Dolin ..356/99 [4 11 Oct;24,1972

Primary Examiner-Ronald L. Wibert Assistant Examiner-V. P. McGrawAttorney-Daniel T. Anderson, Donald R. Nyhagen and Jerry A. Dinardo [57]ABSTRACT A spectrometer and method of spectroscopic analysis forproviding spectral data concerning a scene by collecting and collimatingradiation from the scene to provide a radiation beam dispersing theradiation of the beam into a spectrum, forming a focused image of thespectrum, scanning the spectrum image in rapid succession by multiplenarrow radiation receiving areas, and generating an electrical outputrepresenting the radiant flux incident on each area during its relativescanning movement along the image. The spectrometer may be equipped withmeans for mounting radiation filters over selected radiation receivingareas to provide information concerning selected characteristics, suchas relative polarization, of the radiation from the spectrum image andfor mounting narrow band-pass calibration filters over alternateradiation receiving area to permit calibration of the spectral dataobtained from the scene.

11 Claims, 5 Drawing: Figures PHOTOCELL i 38 RECORDER 2e Patented Oct.24,1972 3,100,331

Fig. l

CONSTANT SPEED MOTOR RECORDER Peter G. WhIre a 440/ 460) INVENTOR.

. 2 2 BY /(d D 2 M r ATTORNEY SPECTROMETER WITH ROTARY SCANNING DISC ANDMETHOD OF SPECTROSCOPIC ANALYSIS t BACKGROUND OF THE INVENTION 1. Fieldof the Invention This invention relates generally to the field ofspectroscopy and more particularly to a novel spectrometer and method ofspectroscopic analysis.

2. Prior Art As will become evident from the ensuring description, thespectrometer and spectroscopic method of the invention are capable ofvaried useful applications. However, the invention is primarilyconcerned with and will be disclosed in relation to measurement, from anaircraft or spacecraft, of small differences in the spectralcharacteristics of radiation flux reflected from a large body of water.

In recent years, there has been a steadily growing interest in thenature and measurement of the color of the worlds oceans. This interesthas been stimulated by extensive investigations which have been carriedout at various oceanographic institutions. In recent years, theopportunity to make measurements of the spectral characteristics ofocean waters from space has become a real possibility which has addedimpetus to ocean color mapping applications.

Few characteristics of the ocean can be measured remotely from anaircraft or spacecraft, and those characteristics which can be someasured, should be investigated fully to determine their usefulness tothe fisheries industry, oceanographers, maritime transport, marinebiologists, meteorolgists, and others. The characteristics which areamenable to such remote measurements are sea state, surface temperature,and the color of spectral characteristics of the upper layers of theocean. These spectral characteristics can define such phenomena asconcentration of water pollution and phytoplankton, and shown theboundaries of areas of upwelling.

SUMMARY OF THE INVENTION The present invention provides a spectrometerand a method of spectroscopic analysis which are ideally suited to theocean color mapping application discussed above. As noted earlier,however, and as will become evident from the ensuing description, thepresent spectrometer and spectroscopic method are not limited inusefulness to this particular application.

The spectrometer of the invention is equipped with beam forming meansfor collecting and collimating radiation from a scene, such as oceanwater, to provide a beam of radiation from the scene. This radiationbeam is transmitted to a dispersion means, such as a diffractiongrating, for dispersing the radiation of the beam into a spectrum whichis then formed into a focused spectral image. Means are provided forscanning this spectral image in rapid succession by a number of narrowradiation receiving areas and generating an electrical signalrepresenting the radiant flux incident on each area from theimage'during relative scanning movement of the respective area along theimage. The successive signals thus generated in response to scanning ofthe spectral image by the successive radiation receiving areas representthe spectral characteristics of the radiation from the scene being 2analyzed. In the particular ocean color mapping application referred toabove, the spectrometer is installed tends tangentially to the slit row.

in an aircraft or spacecraft which follows a selected flight path overthe ocean. The electrical output from the spectrometer providescontinuous spectral information concerning the varying radiation fluxfrom the ocean water along this flight path. This electrical output maybe recorded in any convenient manner for subsequent analysis.

In a particular embodiment of the invention selected for illustration,the radiation receiving areas that scan the spectrum image are radialslits arranged in a con-' centric circular row on a scanning disc whichis otherwise opaque to the radiation being analyzed and is driven inrotation on an axis passing through the center of the slit row. Thespectrum image is focused in the plane of the slits in a manner suchthat the image ex- Accordingly, during rotation of the disc, the slitsundergo lateral scanning movement in succession along the spectral imagefrom one end to the other. Mounted behind the disc to receive radiationfrom the spectrum image through each slit during its scanning movementalong the image is a radiation detector, such as a photomultiplier. Theoutput from this photomultiplier, therefore, is an electrical signalrepresenting the radiant flux from the spectral image which passesthrough each slit during its scanning movement along the image.

According to one unique feature of the invention, narrow bandpassinterference filters may be mounted over alternate slits of the scanningdisc to provide calibration information for calibrating the spectraldata obtained from the scene being analyzed. In the disclosed inventionembodiment, for example, these calibration filers are red and blueinterference filters which are mounted over alternate slits in thescanning disc in a manner such that each filter covers approximately onehalf the disc. The filters produce spikes in the electrical signaloutput from the spectrometer which can be used to spectrally calibratethe data obtained from the instrument.

According to another feature of the invention, the scanning disc may beprovided with means for mount ing selected radiation filters over atleast some of the remaining slits in the disc to provide informationconcerning selected characteristics of the radiation in the spectrumimage. For example, polarizing filters may be used to give informationconcerning the relative polarization of the radiation.

BRIEF DESCRIPTION ON THE DRAWINGS In the drawings:

FIG. 1 diagrammatically illustrates a spectrometer according totheinvention;

FIG. 2 is a view taken on line 22 of FIG. 1;

FIG. 3 is a view taken on line 3-3 in FIG. 1;

FIG. 4 is an enlargement of area 4-4 in FIG. 3; and

FIG. 5 illustrates a spectral data record produced by the instrument.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The drawings illustrate aspectrometer 10 according to the information for providing spectralinformation concerning a scene, such as ocean water. The spectrometer isequipped with beam forming means 12 for collecting and collimating intoa beam B radiation from the scene. Located in the path of the beam B aredispersion means 14 for dispersing the radiation of the beam into aspectrum. This spectrum is transmitted to imaging means 16 which formthe spectrum into a focused image I within an image plane P. Thespectral image is scanned by scanning means 18. Scanning means 18includes a number of spaced radiation receiving areas 20 of relativelynarrow width compared to the length of the spectrum image, and means 22for effecting relative scanning movement of the spectrum image and theradiation receiving areas in a manner to cause lengthwise scanning ofthe image by the areas in rapid succession. Associated with the scanningmeans 18 are radiation sensing means 24 for providing an electricalsignal representing the radiation incident on each radiation receivingarea 20 during its relative scanning movement along the spectrum image.

During operation of the spectrometer 10, the scanning means 18 areoperated to cause relative scanning movement of the radiation receivingarea 20 along the spectral image I in rapid succession. The radiationsensing means 24 generates an electrical output signal representing thevarying radiation from the spectrum image incident on each area duringits relative scanning movement along the spectrum image. The output fromthe radiation sensing means, therefore, is effectively a succession ofperiodic electrical signals representing the radiation incident on thesuccessive radiation receiving areas as they scan the spectrum image.The signal output from the sensing means is fed to a suitable recorder26, such as a magnetic recorder or strip-chart recorder, for recordingthe signal output in a form which may be subsequently used for analyzingthe spectral data obtained from the scene being analyzed.

Referring in greater detail to the particular embodiment of theinvention selected for illustration, the beam forming means 12 comprisean objective lens 28 for collecting radiation from the scene. Locatedbehind and in a focal plane of this lens is a mask 30 which is opaque tothe radiation and is formed with a radiation transparent entrance slit32 on the optic axis of the lens. Lens 28 forms an image of the scene inthe plane of the entrance slit 32. The beam forming means 12 furthercomprise optical means 34 on the axis of lens 28 and slit 32 whichreceives and collimates the radiation from the image and transmits theincident radiation, as the radiation beam B, to the dispersion means 14.In the particular inventive embodiment illustrated, the opticalcollimating means 34 comprises a portion of a spherical mirror 36,another portion of which comprises the imaging means 16. The particulardispersion means 14 shown is a diffraction grating located on and in aplane normal to the optic axis of the mirror 36. The optic axis of theobjective lens 28 and entrance slit 32 parallels and is spaced laterallyfrom the optic axis of the mirror, as shown, in a manner such thatradiation from the scene which enters the spectrometer through the lensis reflected from the surface portion 34 of the mirror to thediffraction grating 14. This grating disperses the radiation of thecollimated beam 8 into a spectrum. This spectrum is reflected to thesurface portion 16 of the mirror 36 and then back toward the image planeP along an axis parallel to the axes of the objective lens 28 and mirror36. The surface portion 16 of the mirror brings the spectrum to focus inthe image plane to produce in the latter plane the focused spectralimage I referred to earlier.

The spectrum image scanning means 18 of the illustrated spectrometer 10comprises a rotary scanning disc 38 which is opaque to the radiation ofthe spectrum image I. This disc is located in the image plane P and hasits central rotation axis laterally spaced from the optic axis alongwhich the spectrum image is transmitted from the mirror 36 to the imageplane. The disc projects radially across the latter axis so that thespectrum image is formed on the disc at a position radially spaced fromits rotation axis. The scanning driver 22 is a constant speed motor,such as a synchronous motor, which is coupled to the scanning disc 38 todrive the latter in rotation on its central axis. Formed in the disc area number of radial slits which are transparent to the radiation of thespectrum image I and constitute the radiation receiving areas 20. Radialslits 20 are arranged in a circular row concentric with the centralrotation axis of the disc 38 and are uniformly circumferentially spacedabout the disc. As will appear from the ensuing description, thescanning disc may contain any number of slits providing that the slitsare so spaced that the spectrum image is never scanned by more than oneslit at a time. The scanning disc of a typical spectrometer according tothe invention may contain 15 equally spaced slits.

The entrance slit 32, diffraction grating 14, and mirror 36 are soarranged relative to the scanning disc 38 that the spectral image I isformed in the plane of an in tangential relation to the circular centerline of the slits 20. Accordingly, during rotation of the scanning disc38, the slits 20 undergo lateral scanning movement in succession alongthe spectrum image from one end to the other. The radial length of eachslit is somewhat less than the transverse width of the spectrum image,such that each slit is fully illuminated by the image during the entirescanning movement of the slit from one end of the image to the other.

The radiation sensing means 24 of the illustrated spectrometer comprisesa radiation detector or photocell, such as a photomultiplier, mountedbehind the disc 38 on an axis normal to the disc and intersecting thecircular center line of the slit row. From this description, it will beunderstood that during rotation of the scanning disc 38, the photocell24 receives radiation from a narrow band of the spectrum image I, whichband progresses along the image from one end to the other. The radiationincident on the photocell and hence the electrical signal fed from thephotocell to the recorder 26, thus vary in accordance with the intensityof the successive spectral lines in the image. All but one of the slits20 have a uniform width substantially less than the length of thespectrum image. In a typical spectrometer according to the invention,for example, the slits are 50 to Angstrom units in width.

Recorder 26 produces a record 40, which is shown to be a strip recordingconsisting of successive traces 42 each representing the varyingradiation intensity along the spectrum image. In the particular oceancolor mapping application referred to earlier, the spectrometer ismounted in an aircraft or spacecraft which is flown along a selectedflight path passing over an ocean. In this application, the spectrumimage, and hence the traces produced by the recorder 26, continuouslychange along the flight path in accordance with the changing spectralcharacteristics of the ocean water along the flight path. The record 40may be subsequently analyzed to determine the spectral characteristicsof the ocean water at any point along the flight path and to map theocean color along the flight path.

Accurate assessment of the spectral data obtained with the presentspectrometer requires spectral calibration of the data. This calibrationinvolves both intensity calibration and wavelength calibration.Intensity calibration is accomplished by measuring and/or indicating onthe data record 40 produced by the recorder 26 the output signal level,of the photocell 24 when the instrument receives radiation from a knownsource, such as a grey card of known reflectivity illuminated bysunlight. Wavelength calibration is accomplished by providing the datarecord with indicators or reference points along each successivespectral trace 42 of the record defining positions on the tracecorresponding to known wavelengths. Preferably, these reference pointsare located near the ends of each spectral trace. Assuming lineardispersion of the spectrum by the grating 14, the wavelengthcorresponding to any point along the spectral trace can then bedetermined.

In the particular embodiment of the invention illustrated, wavelengthcalibration is accomplished by mounting narrow bandpass interferencefilters 44, 46 over alternate slits 20, hereinafter referred to ascalibration slits in the scanning disc 38. Each filter coversapproximately one-half its respective slit. As each calibration slitscans across the spectrum image I, its interference filters block thepassage of radiation through the slit to the photocell 24 until the slitis aligned with the portions of the spectrum image corresponding to therespective transmission frequencies of the filters. The filters thentransmit radiation from the spectrum image to the photocells to producecalibration spikes 44a, 46a, respectively, on the data record 40.

From the above discussion, it will be understood that the data recordproduced by the illustrated spectrometer will resemble the record 40illustrated in FIG. 5. This data record comprises a succession of spacedspectral traces 42 produced by scanning movement along the spectralimage of the intervening slits 20 in the scanning disc 38 between thecalibration slits. Within the gaps between the spectral traces are thecalibration spikes 44a, 46a produced by scanning movement of thecalibration slits along the spectrum image. Since the several slits 20in the scanning disc 38 are uniformly spaced around the disc and thelatter rotates at a constant speed, a reference point P halfway betweeneach point of adjacent calibration spikes 44a represents the position,along the intervening spectral trace 42, corresponding to .thetransmission frequency band of the interference filter 44. Similarly, apoint P halfway between each pair of adjacent calibration spikes 46arepresents the position, along the intervening spectral trace,corresponding to the transmission frequency band of the filter 46. Asnoted above, the frequency corresponding to any other point of thespectral trace may then be determined from 6 these two reference points.Maximum calibration accuracy is obtained by using interference filters44, 46,

such as red and blue filters, which provide reference points adjacentthe extremities of each spectral trace. These filters should have anarrow transmission frequency band on the order of 15 Angstrom units orless.

According to another unique feature of the invention, the scanningdisc38 is equipped with means for mounting additional filters 48 ofknown radiation transmission characteristics over one or more of theintervening slits 20 between the calibration slits. For example,polarizing filters may be mounted over certain of the intervening slitsto provide information concerning the relative polarization of theradiationqln order to determine the spectral trace corresponding to aselected filter 48, one of the intervening slits 20 in the scanning disc38, i.e., slit 20a in the illustrated disc, may be left uncovered andenlarged relative to the other slits. The periodic spectral tracesproduced by scanning movement of this enlarged slit along the spectrumimage may be readily distinguished from, the remaining spectral tracesbecause of their greater magnitude, and the spectral trace produced by aslit covered by a particular filter may be determined by counting.

The operation of the illustrated spectrometer 10 is obvious from theforegoing description. It should be noted here that-calibration methodsother than those described above, may be employed to spectrallycalibrate the data obtained with the present instrument. What is claimedas new in support of Letters Patent 1s:

1. A spectrometer for providing spectral information concerning a scenecomprising:

an opaque rotary scanning disc having at least two circumferentiallyspaced radial scanning slits equally spaced from the rotation axis ofthe disc; beam forming means for collecting and forming into a beamradiation from said-scene; dispersion means for dispersing the radiationof said beam into a spectrum; imaging means for forming a focused imageof said spectrum on said disc over the circular path of rotation of saidslits with said disc in a manner such that the spectral lines of saidimage extend generally radially of the disc whereby said slits undergolateral scanning movement in succession across said image duringrotation of said disc; radiation sensing means behind said disc forreceiving and generating an electrical signal representing the radiationincident on each scanning slit during its scanning movement along saidimage; one of said slits being totally transparent to said radiation,whereby said one slit transmits to said sensing means all of theradiation incident on the latter slit during its scanning movement alongsaid image; and narrow band pass filter means across the other slitwhich transmits radiation only within at least one narrow band of saidspectrum. 2. A spectrometer according to claim 1 wherein: said filtermeans comprises a pair of narrow bandpass filters each occupyingapproximately one-half of the respective slit; and

said filters having transmission frequency bands near the ends,respectively, of said radiation spectrum.

3. A spectrometer according to claim 1 wherein:

said scanning disc contains a number of said scanning slits arranged ina circular row centered on said rotation axis and narrow band passfilter means across alternate slits which transmit radiation only withinat least said one narrow band of said spectrum; and

at least some of the remaining intervening slits being totallytransparent to said radiation.

4. A spectrometer according to claim 3 wherein:

each said filter means comprises a pair of narrow band pass filters,each occupying approximately one-half of the respective slit; and

said filters having transmission frequency bands near the ends,respectively of said radiation spectrum.

5. A spectrometer according to claim 3 including:

means for mounting over one of said transparent slits a filter havingselected radiation transmission characteristics.

6. A spectrometer according to claim 3 wherein:

all of said slits except one are of substantially equal size and saidone slit is enlarged.

7. A spectrometer according to claim 6, wherein:

said enlarged slit is a transparent slit.

8. A spectrometer according to claim 1 wherein:

said scanning disc contains a number of said scanning slits arranged ina circular row centered on said rotation axis and narrow band passfilter means across alternate slits which transmit radiation only withinat least said one narrow band of said spectrum;

at least some of the remaining intervening slits being totallytransparent to said radiation;

each said filter means comprises a pair of narrow bandpass filters eachoccupying approximately one-half of the respective slit;

said filters having transmission frequency bands near the ends,respectively, of said radiation spectrum;

means for mounting over one of said transparent slits a filter havingselected radiation transmission characteristics; and

all of said slits except one of said transparent slits are ofsubstantially equal size and said one transparent slit is enlarged.

9. A spectrometer for providing spectral information concerning a scene,comprising:

beam forming means for collecting and collimating into a beam radiationfrom said scene, said beam forming means including a radiation opaquemask having a radiation transparent entrance slit, and objective lens infront of said slit for producing an image of said scene in the plane ofsaid slit, a con- ;cave spherical mirror behind said slit for receivingradiation from said image and reflecting the incident radiation in theform of a collimated radiation beam;

a diffraction grating positioned in front of said mirror on the axis ofsaid beam for dispersing the radiation of said beam into a spectrum andreflecting said spectrum back to said mirror;

said mirror reflecting said spectrum toward an image plane and formingin said plane a focused image of said 8 t t a rotary radiation opaquescanning dlSC positioned in said plane and containing a number of radialradiation transparent slits arranged in a circular row concentric withthe rotation axis of said disc;

said slits being uniformly spaced along said row and said spectrum imagebeing formed over and in tangential relation to said row, wherebyrotation of said disc is effective to cause lateral scanning movement ofsaid slits in rapid succession along said spectrum image;

means for driving said disc in rotation;

radiation sensing means positioned behind said disc opposite saidspectrum image for receiving radiation from said image through each slitduring its scanning movement along said image;

narrow band pass filter means across alternate slits which transmitradiation only within at least one narrow band of said spectrum;

at least some of the remaining intervening slits being totallytransparent to said radiation; and

all of said slits except one are of substantially equal size and saidone slit is enlarged.

10. A spectrometer according to claim 9 including:

means on said disc for mounting a selected radiation filter over onetransparent slit.

1 1. A spectrometer according to claim 9 wherein:

each filter means comprises a pair of narrow bandpass filters eachoccupying approximately one-half its respective slit; and

said filters having transmission frequency bands near the ends of saidradiation spectrum.

1. A spectrometer for providing spectral information concerning a scenecomprising: an opaque rotary scanning disc having at least twocircumferEntially spaced radial scanning slits equally spaced from therotation axis of the disc; beam forming means for collecting and forminginto a beam radiation from said scene; dispersion means for dispersingthe radiation of said beam into a spectrum; imaging means for forming afocused image of said spectrum on said disc over the circular path ofrotation of said slits with said disc in a manner such that the spectrallines of said image extend generally radially of the disc whereby saidslits undergo lateral scanning movement in succession across said imageduring rotation of said disc; radiation sensing means behind said discfor receiving and generating an electrical signal representing theradiation incident on each scanning slit during its scanning movementalong said image; one of said slits being totally transparent to saidradiation, whereby said one slit transmits to said sensing means all ofthe radiation incident on the latter slit during its scanning movementalong said image; and narrow band pass filter means across the otherslit which transmits radiation only within at least one narrow band ofsaid spectrum.
 2. A spectrometer according to claim 1 wherein: saidfilter means comprises a pair of narrow bandpass filters each occupyingapproximately one-half of the respective slit; and said filters havingtransmission frequency bands near the ends, respectively, of saidradiation spectrum.
 3. A spectrometer according to claim 1 wherein: saidscanning disc contains a number of said scanning slits arranged in acircular row centered on said rotation axis and narrow band pass filtermeans across alternate slits which transmit radiation only within atleast said one narrow band of said spectrum; and at least some of theremaining intervening slits being totally transparent to said radiation.4. A spectrometer according to claim 3 wherein: each said filter meanscomprises a pair of narrow band pass filters, each occupyingapproximately one-half of the respective slit; and said filters havingtransmission frequency bands near the ends, respectively of saidradiation spectrum.
 5. A spectrometer according to claim 3 including:means for mounting over one of said transparent slits a filter havingselected radiation transmission characteristics.
 6. A spectrometeraccording to claim 3 wherein: all of said slits except one are ofsubstantially equal size and said one slit is enlarged.
 7. Aspectrometer according to claim 6, wherein: said enlarged slit is atransparent slit.
 8. A spectrometer according to claim 1 wherein: saidscanning disc contains a number of said scanning slits arranged in acircular row centered on said rotation axis and narrow band pass filtermeans across alternate slits which transmit radiation only within atleast said one narrow band of said spectrum; at least some of theremaining intervening slits being totally transparent to said radiation;each said filter means comprises a pair of narrow bandpass filters eachoccupying approximately one-half of the respective slit; said filtershaving transmission frequency bands near the ends, respectively, of saidradiation spectrum; means for mounting over one of said transparentslits a filter having selected radiation transmission characteristics;and all of said slits except one of said transparent slits are ofsubstantially equal size and said one transparent slit is enlarged.
 9. Aspectrometer for providing spectral information concerning a scene,comprising: beam forming means for collecting and collimating into abeam radiation from said scene, said beam forming means including aradiation opaque mask having a radiation transparent entrance slit, andobjective lens in front of said slit for producing an image of saidscene in the plane of said slit, a concave spherical mirror behind saidslit for receiving radiation from said image and reflecting the incidentRadiation in the form of a collimated radiation beam; a diffractiongrating positioned in front of said mirror on the axis of said beam fordispersing the radiation of said beam into a spectrum and reflectingsaid spectrum back to said mirror; said mirror reflecting said spectrumtoward an image plane and forming in said plane a focused image of saidspectrum; a rotary radiation opaque scanning disc positioned in saidplane and containing a number of radial radiation transparent slitsarranged in a circular row concentric with the rotation axis of saiddisc; said slits being uniformly spaced along said row and said spectrumimage being formed over and in tangential relation to said row, wherebyrotation of said disc is effective to cause lateral scanning movement ofsaid slits in rapid succession along said spectrum image; means fordriving said disc in rotation; radiation sensing means positioned behindsaid disc opposite said spectrum image for receiving radiation from saidimage through each slit during its scanning movement along said image;narrow band pass filter means across alternate slits which transmitradiation only within at least one narrow band of said spectrum; atleast some of the remaining intervening slits being totally transparentto said radiation; and all of said slits except one are of substantiallyequal size and said one slit is enlarged.
 10. A spectrometer accordingto claim 9 including: means on said disc for mounting a selectedradiation filter over one transparent slit.
 11. A spectrometer accordingto claim 9 wherein: each filter means comprises a pair of narrowbandpass filters each occupying approximately one-half its respectiveslit; and said filters having transmission frequency bands near the endsof said radiation spectrum.